1. Field of the Invention
This invention relates to an optical polarization-state converting apparatus which can be applied to an optical isolator for use in for example, a semiconductor laser device utilized in optical communications equipment, information processing systems, measurement instruments, etc., for preventing noise caused by return light. The present invention also relates to a semiconductor laser device including such an optical isolator, and to an optical modulator including such an optical isolator.
2. Related Background Art
In recent years, the speed of optical data transmission has become faster and faster, and the capacity of information recording has become greater and greater. For this purpose, optical fiber communication and the optical disc memory have been developed. As optical communication systems, there exist medium and long distance transmission systems wherein the long distance systems use wavelengths of 1.3 or 1.5 .mu.m, and the short or medium distance systems (e.g. a local area network (LAN) and so forth) which use wavelengths of 0.8 .mu.m.
In either case, light from a semiconductor laser is condensed onto the end surface of an optical fiber and is propagated through the fiber. In this case, however, light reflected by end surfaces of the fiber and other optical components returns to the active layer of the semiconductor laser, and as a result the oscillation of the semiconductor laser becomes unstable and fluctuations of power and wavelength occur. Particularly, in a distributed Bragg reflection type laser diode(DBR-LD), there occur such large fluctuations that a single mode is sometimes changed to a multimode. Further, in coherent optical communications systems which are proposed for future communication systems, negative influences caused by such return light are especially disadvantageous since only the phase is changed without on/off modulation of the light.
In optical disc memories, light reflected from a disc substrate in addition to that reflected from optical components returns to the semiconductor laser, and induces the above-noted unstable oscillation and noise problems. The occurrence of such noise will increase the error rate of transmission signals in an optical communication system and will degrade the signals reproduced from the optical disc memory.
The optical isolator is the only device for preventing the return light from reaching the semiconductor laser. The optical isolator has an irreversible or nonreciprocal transmittance property, and completely cuts off the return light to prevent the return light from reaching the active layer in the semiconductor laser. Presently, the optical isolator has the following two problems.
One is the problem of a wavelength range. As shown in FIG. 1, the conventionally used optical isolator utilizes a magnetic garnet single crystal 1 such as YIG as a Faraday rotating element. The absorption edge of garnet lies at about 1 .mu.m of wavelength. So the garnet is transparent for the wavelength range of 1.3 or 1.5 .mu.m which is used in the long distance optical communication system, but the light absorption of garnet is large for light in the wavelength range (0.8 .mu.m band) which is used in the short distance optical communication system or in the optical disc memory. Therefore, in the wavelength range of 0.8 .mu.m, only an optical isolator in which a .lambda./4 plate and a polarization beam splitter are combined is used. The isolation ratio obtained by this isolator is at most only about 20 dB. The isolation ratio would be smaller if the plane of polarization in the return light will be changed. Further, this type of isolator cannot be used in the system in which the rotation (i.e., Kerr rotational angle or Faraday rotational angle) of the polarization plane in light reflected by or transmitted through a disc (such a magneto-optical disc) is detected as a signal.
The other problem is that the conventional isolator is hard to integrate with other optical devices. The optoelectronic integrated circuit (OEIC) or optical integrated circuit has been developed for obtaining such devices that have high-speed operation and high efficiency. In these devices, GaAs, InP and the like are formed in a compound semiconductor substrate to produce optical devices such as semiconductor lasers. For this purpose, it is required to integrate the optical isolator on the compound semiconductor substrate. While a film of the magnetic garnet itself can be grown by liquid phase epitaxy or sputtering, a magnetic garnet film of high quality cannot be grown on such a substrate since the magnetic garnet differs from the GaAs or InP substrate in lattice constant and in coefficient of thermal expansion. Thus, it is difficult to integrate the optical isolator with other optical elements so long as the magnetic garnet film is used as the Faraday material.
As is known from the above, an optical isolator which can operate in a short wavelength range (i.e., 0.8 .mu.m band) and can be easily integrated with other optical devices would be desirable.
To achieve this purpose, there is disclosed in Japanese Patent Application No. 2-292887, or an integrated-type optical isolator that uses a magnetic semiconductor as the Faraday material, as shown in FIG. 2. It is known that in the magnetic semiconductor CdMnTe the site of Cd in the II-VI group compound semiconductor CdTe is replaced by Mn, and is transparent for the visible range of light and has a great Faraday rotation angle. CdTe can be grown on a GaAs substrate by molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) as a high-quality film. As shown in FIG. 2, such an isolator comprises a CdTe buffer layer 10, a cladding layer 11, a waveguide layer 12 and a cladding layer 13. These elements of the optical isolator 14 can be formed on a common substrate 15 together with a DBR laser part 16, which includes a lower cladding layer 17, an active layer 18, an upper cladding layer 19, a waveguide layer 20, a cladding layer 21, a contact layer 22, corrugations 23, a p-side electrode 24, an n-side electrode 25, and a metal film 26 for acting as a polarizing plate.
The integrated-type isolator using the magnetic semiconductor CdMnTe has solved the above-discussed problems, but this isolator also has the following problem.
Phase matching is needed between the transverse electric (TE) wave and transverse magnetic (TM) wave in order to produce an effective waveguide type optical isolator as shown in FIG. 2. FIGS. 3A-3C illustrate the way to obtain phase matching. When a laser light propagates through a film in a z-direction as shown in FIG. 3A, the refractive index n.sub.TE for the TE wave is larger than the refractive index n.sub.TM for the TM wave when the film thickness h.sub.o is finite. This difference is illustrated in FIG. 3B, and is due to shape double refraction. Provided that the propagation constant difference between the TE and TM waves is .DELTA..beta., then EQU .DELTA..beta.=2.pi./.lambda..multidot.(n.sub.TE -n.sub.TM) (1)
The mode conversion rate R from TE wave to TM wave is given by EQU R=.theta..sub.F.sup.2 /{.theta..sub.F.sup.2 +(.DELTA..beta./2).sup.2 }.multidot. sin.sup.2 [{.theta..sub.F.sup.2 +(.DELTA..beta./2).sup.2 }.sup.1/2 .multidot.l] (2)
where .theta..sub.F is a Faraday rotation angle per unit length and l is the waveguide length. From this, it is learned that the condition .DELTA..beta.=0 or n.sub.TE =n.sub.TM is necessary in order to increase the mode conversion rate. Here, since the mode is quantized by the waveguide film, rotation of the polarization plane in a propagating light is exhibited by the mode conversion rate.
The equation (2) is derived according to, e.g., a method described in "Optical Waves In Crystal" written by A. Yariv and P. Yeh, published by John Wiley & Sons, as follows: EQU .gradient..times.E+.differential.B/.differential.t=0 (2-1) EQU .gradient..times.H+.differential.D/.differential.t=J (2-2) EQU .gradient..multidot.D=.rho. (2-3) EQU .gradient..multidot.B=0 (2-4)
From Maxwell equations represented by the formulas (2-1)-(2-4), the following wave equation given by a formula (2-5) is derived EQU .gradient..times.(1/.mu..gradient..times.E)+.epsilon..differential..sup.2 E/.differential.t.sup.2 =0 (2-5)
Provided that E .varies. exp(i .omega.t) is satisfied in the formula (2-5), the following formula (2-6) is obtained EQU .gradient..times.(.gradient..times.E)-.omega..sup.2 .mu.(.epsilon.+.DELTA..epsilon.)E=0 (2-6)
From the formula (2-6), mode equations represented by the following formulas (2-7) and (2-8) are derived EQU dA.sub.1 /d.zeta.=(i.omega..sup.2 .mu.)/(2k.sub.1).multidot.{.DELTA..epsilon..sub.11 A.sub.1 +.DELTA..epsilon..sub.12 A.sub.2 exp[-i(k.sub.1 -k.sub.2).zeta.]}(2-7) EQU dA.sub.2 /d.zeta.=(.omega..sup.2 .mu.)/(2k.sub.2).multidot.{.DELTA..epsilon..sub.21 A.sub.1 exp[i(k.sub.2 -k.sub.1).zeta.]+.DELTA..epsilon..sub.22 A.sub.2 } (2-8)
The formula (2) is derived by substituting diagonal components (.DELTA..epsilon..sub.11, .DELTA..epsilon..sub.22)=0 and off-diagonal components .DELTA..epsilon..sub.12 /.epsilon.=2n .DELTA.n-iG, .DELTA..epsilon..sub.21 /.epsilon.=2n .DELTA.n+iG into the above formulas (2-7) and (2-8).
For this purpose, as is shown in FIG. 3C, attempts have been made to achieve the relation n.sub.TE =n.sub.TM at a certain film thickness h.sub.o by causing the waveguide to have an anisotropic property in a direction normal to the film surface (x-axis). In the case of garnet film, attempts have been made to bring forth a strain-induced double refraction property utilizing the lattice constant difference between the substrate and film (waveguide), or a growth-induced double refraction property by controlling the temperature and fraction mole in the process of film growth. Further, there is a technique for forming a grating along a light propagating direction in order to compensate for the propagation constant difference.
However, the above methods require strict control of the growth and processing conditions, and once those methods were implemented, processing adjustments would be virtually impossible. Thus, those methods are believed to be impractical.
In addition to the optical isolator, another important device is an optical modulator for increasing the bit rate in an optical communications system, and for enhancing the write-in and read-out speeds in optical disc memories.
When the semiconductor laser is modulated at a high speed in optical communications systems, the spectrum width of unwanted oscillated lights will be widened and long distance transmission will become difficult due to the wavelength dispersion in the fibers. To overcome this drawback, a method has been developed in which the semiconductor laser is dc-driven and light therefrom is pulse-modulated by an external optical modulator.
FIG. 4 shows an example in which the semiconductor laser and optical modulator are integrated (see, for example, Jpn. J. Appl. Phys. vol. 24(1985)L442). A laser part 30 and a modulator part 31 have the same p-i-n structure. These parts 30 and 31 are electrically separated by a groove 32, but waveguides of the parts 30 and 31 are optically coupled. Therefore, light emitted from the laser part 30 is incident upon the waveguide of the modulator part 31, and emerges after propagating a given distance. When the light propagates through the waveguide of the modulator part 31, the light quantity is reduced due to light absorption. The absorption coefficient of a multiple quantum well (MQW) layer or the waveguide can be modulated by controlling the application of a reverse-bias with a quantum confinement Stark effect (QCSE). As a result, the intensity of the emerging light from the modulator part 31 can be modulated.
In present optical systems depicted in FIG. 5, the above-discussed two devices (optical modulator 41 and isolator 42) must be serially arranged along the light path. Therefore, the insertion loss is twice as much, and the size of the components becomes large. Further, when those devices are to be integrated with the semiconductor laser 43, devices having different layer structures must be fabricated on a common substrate, as a result of which the process for fabrication will be complicated. Thus, it is desired to provide a single device which acts as an optical isolator and a modulator, and can be integrated with a semiconductor laser.